This application claims the priority of German patent document 199 11 909.0, filed Mar. 17, 1999, and European patent document EP 99113350.5, filed on Jul. 9, 1999, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to an apparatus for fractionating a gas mixture at low temperature comprising (1) a separation column; (2) a heat-exchanger block, which possesses a main heat-exchanger section and a condenser/evaporator section, wherein the condenser/evaporator section has evaporation passages and condensation passages; (3) a first feed gas line for supplying feed gas to the main heat-exchanger section; (4) a second feed gas line, for introducing cooled feed gas into the separation column; (5) a first liquid line which leads from the lower region of the separation column to the inlet of the evaporation passages; (6) a gas line which leads from the upper region of the separation column to the condensation passages; and (7) a return line to introduce condensate which is formed in the condensation passages into the upper region of the separation column.
The most important area of application of the present invention is the low-temperature fractionation of air in single- or multiple-column processes, especially the production of nitrogen from air in a single-column process. "Separation column" is here taken to mean a conventional mass-transfer column which comprises rectification plates, packing (random packing) and/or arranged packing as mass-transfer elements, in particular a rectification column or a distillation column.
In FIG. 3 of JP-A-10206012, the main heat exchanger and condenser/evaporator are not formed, as is generally customary, by separate heat-exchanger blocks, but are integrated in one heat exchanger block which has one main heat-exchanger section for cooling air against return streams and one condenser/evaporator section for producing return liquid by vaporizing the bottoms liquid of the separation column. This integrated type of construction has the advantage of lower plant costs in comparison with conventional plants.
The liquid which is vaporized in the heat-exchanger block of the plant in accordance with JP-A-10206012, in addition to comprising the main components oxygen, nitrogen and argon, also comprises those air constituents that are less volatile than oxygen and that are not removed from the feed air during the air purification upstream of the main heat-exchanger section. During the vaporization of the oxygen-enriched bottoms liquid from the separation column in the heat exchanger block according to JP-A-10206012, there is the risk that some of these less volatile constituents do not vaporize completely but accumulate in the liquid which is present in the condenser/evaporator section. In the event of such accumulations, for example of hydrocarbons, a great safety risk would be expected. Interruptions to operations to remove the less volatile components from the condenser/evaporator section would make continuous long-term operation of the plant impossible. This would mean a high operating expenditure and significant losses of production. Therefore, in practice, the use of such integrated heat exchangers has been avoided.
The object underlying the present invention is to provide an apparatus of the type mentioned at the outset and a corresponding process which are more expedient to operate, in particular in a particularly safe and economical manner.
This object is achieved by a phase-separation device that is connected on one side to the outlet of the evaporation passages and on the other side to a second liquid line which leads from the phase-separation device to the inlet of the evaporation passages and in addition has a connection to a purge line.
The apparatus of the present invention makes possible reliable operation of the integrated heat-exchanger block without interruption to operations. The liquid is only partially vaporized in the condensor/evaporator section and the resultant vapor is separated in the phase-separation device from the proportion that has remained in the liquid state. One part of the proportion that has remained in the liquid state is returned via the second liquid line to the inlet of the evaporation passages of the condenser/evaporator section and a second part is discarded continuously or batchwise via a purge line.
Dividing the proportion which has remained in the liquid state from the phase-separation device which, in the case of air fractionation is enriched with less volatile components, prevents an unwanted concentration. It is thus possible to eject less volatile impurities and to keep their content in the evaporation passages within a safe limiting value (for example, below 500 ppm CH.sub.4 equivalents). For example, the proportion of the liquid discarded via the liquid line is seven to 15 times, preferably eight to ten times, the amount vaporized in the evaporation passages (the relative amounts relate here and hereinafter to molar amounts). The purge amount discarded via the purge line is, for example, from 0.05 to 0.5%, preferably from 0.1 to 0.2%, of the total amount of gas mixture to be fractionated.
The heat-exchanger block is formed in the present invention preferably by a plate heat exchanger, especially by a brazed aluminum plate heat exchanger. In this case, the main heat-exchanger section is preferably situated above the condenser/evaporator section.
Generally, in the apparatus according to the present invention, only a single heat-exchanger block is used. This can be, for example, fabricated in one piece or can be manufactured by joining together (for example by flanges) two or more sections. However, the invention can also be applied to larger plants by connecting two or more such heat-exchanger blocks in parallel. Each of these heat-exchanger blocks then has both a main heat-exchanger section and a condenser/evaporator section.
The main area of application of the present invention is in single-column plants in which the condenser/evaporator section is preferably the top condenser of the single separation column. However, the invention is also applicable in principle to other processes having two or more columns; for example, the main condenser of a double-column plant can be formed by the condenser/evaporator section.
The phase-separation device can be implemented in various manners. First, the phase-separation device can be formed by a vessel disposed outside the heat-exchanger block, which vessel is connected via a line to the outlet of the evaporation passages. In a second example, the phase-separation device is formed by a collector in the form of a header disposed laterally at the heat-exchanger block; alternatively thereto, a corresponding header can be disposed on both sides of the heat-exchanger block. "Header" is taken to mean a distribution device or collection device which is flow-connected to a defined group of passages of a heat-exchanger block and serves for the feed or take-off of fluid flowing through these passages. The headers mentioned here can be constructed, for example, to be of half-tube shape. In a third variant, the phase-separation device is formed by a region disposed within the heat-exchanger block in the transition between the condenser/evaporator section and the main heat-exchanger section.
For the construction of the liquid line also, there are various alternatives. The liquid line can be disposed outside the heat-exchanger block or be formed by passages within the heat-exchanger block. The second variant is suitable especially when the phase separation is carried out within the heat-exchanger block; for this there can be used, for example, the otherwise unused continuations of the passages for cooling the gas mixture to be fractionated, which passages are interrupted at the lower end of the main heat-exchanger section.
The vapor from the phase-separation device is preferably fed to the main heat-exchanger section at its cold end.
The heat-exchanger block used in the present invention can be used in any process and any plant in which a first fluid is cooled in a main heat-exchanger section and a second fluid is vaporized against a condensing third fluid in a condenser/evaporator section.
The passages for the gas from the upper region of the separation column (in the case of air fractionation, nitrogen) can pass without interruption through the entire length of the heat-exchanger block. In this case, the gas is introduced into the heat-exchanger block via the gas line in the transition region between the main heat-exchanger section and the condenser/evaporator section, wherein a part of the gas flowing upwards into the main heat-exchanger section is heated and taken off as product, another part flowing downwards into the condensation passages of the condenser/evaporator section is liquefied.
The passages for the fraction originating from the lower region of the separation column can be constructed continuously in a similar manner. In particular, if the phase-separation device is disposed within the heat-exchanger block, the vapor formed in the evaporation passages can flow through the main heat-exchanger section remaining in the same throughways.
Preferably, at least one group of passages of the heat-exchanger block is interrupted between main heat-exchanger section and condenser/evaporator section. If the heat-exchanger block is implemented as a brazed aluminum plate heat exchanger, the passages are interrupted by horizontally or obliquely disposed walls (closure strips, side bars), which are disposed in the transition region between main heat-exchanger section and condenser/evaporator section. Walls of this type can close off, for example, the passages for cooling the gas mixture to be fractionated on their lower side and/or the evaporation passages on their upper side. The vapor from the phase-separation device can be introduced into the continuation of the above closed-off evaporation passages in order to heat up in the main heat-exchanger section against feed gas to be cooled.
Preferably, the gas to be condensed is fed to the upper end of the condenser section and flows downwards concurrently with the condensate formed within the condensation passages.
The invention and other details of the invention are described in more detail below on the basis of two illustrative examples shown diagrammatically in the drawings. The examples relate to the production of gaseous nitrogen by low-temperature fractionation of air in single-column plant.